All semiconductor devices have some form of conduction resistance which leads to generation of heat when these devices are operated. The flow of heat starts at the semiconductor junction through which electric current travels. This heat is conducted from the device body onto the package that the semiconductor is housed in, and then to the air. This heat energy increases the temperature of the semiconductor. If the heat is not dissipated from the semiconductor fast enough, the semiconductor temperature may increase beyond the specified operating temperature. A heat sink is an environment or object that absorbs heat from another object using thermal contact. Heat sinks are used to aid in the absorption of heat from the surface of a semiconductor.
FIG. 1 shows an example of a heat sink. The heat producing semiconductor 16 is attached to a substrate 15. A metal lid 19 is placed in close proximity to the semiconductor 16. The metal lid 19 and the semiconductor are brought in thermal contact with the help of thermal interface material 18. The metal heat sink 17 is placed on top of the lid 19, and both the lid 19 and heat sink 17 are brought into thermal contact using thermal interface material 18. The goal of a heat sink is to effectively transfer heat away from the surface of the semiconductor to the surrounding air. The heat transfer can occur in three ways: conduction, convection and radiation. In the example shown in FIG. 1, heat flows from the surface of the semiconductor 16 to the lid 19 through conduction. The transfer of heat from the lid 19 to the heat sink 17 is also through conduction. The heat transfer from the surface of the heat sink 17 to the surroundings is through convection and radiation. Increase in the rate of the above three modes of transfer of heat increases the rate at which heat is transferred away from the semiconductor.
Heat transfer through conduction can be increased by using materials with high conduction coefficient. Metals are good conductors of heat; especially metals like copper, silver and gold. Convection transfers heat energy by currents within the fluid. If the surrounding fluid of the heat sink 17 (shown in FIG. 1) is air, then the heat is transferred due to currents near the air and heat sink interface. If the currents are caused only due to the temperature gradient near the interface of air and the heat sink, then the heat transfer is due to natural convection. If the currents in the fluid are also caused by external forces, then this mode of heat transfer is called forced convection. For example, a fan can be placed above the heat sink 17 to force the air around it and increase the rate of heat transfer.
Passive cooling, for example, use heat pipes that employ phase changing mechanism to transfer heat from the semiconductor to the heat sink fins. A liquid undergoes phase change into vapor by absorbing heat at the evaporator end placed over the semiconductor. The vapor then releases heat at the condenser end, which is in contact with the heat sink fins, by changing its phase back to liquid. Electrical energy is not employed to carry out the passive cooling mechanism. The mechanism occurs naturally when there is a temperature difference between the evaporator and the condenser sites. Active cooling methods, on the other hand, require forcing a fluid, for example magneto-hydro-dynamic (MHD) fluid, over a hot surface in order to cool it. The fluid is usually forced by means of a pump. The pump uses external energy, for example electrical energy, for its operation. The moving fluid absorbs heat from the semiconductor and dissipates the heat to its surroundings. The fluid may also be made to make thermal contact with a heat sink to enhance heat dissipation. FIG. 2 shows two cooling devices, the passive heat pipe cooling (A) and active liquid cooling (B). Both cooling methods have their own advantages and disadvantages. With regard to passive cooling, the method does not consume external energy for its operation. Although heat pipe passive cooling method is currently quite effective in cooling high power semiconductors, the increasing power dissipation of future semiconductors may render this passive cooling method inadequate due to physical limitations of size and rate of heat transfer.
Due to their high heat transfer rates, active cooling methods are generally very efficient in cooling high power dissipating semiconductors. However, active cooling methods consume external power for their operation. Thus, in instances when the power supply to the active cooling components is interrupted or fails, the semiconductor may reach undesirable temperatures.